The present invention relates to a correcting circuit of the spool-type distortation for displaying units that consists of image tubes, and more particularly to a correcting circuit of the spool-type distortion for television receivers having non-spherical surface of CRT fluorescent screens.
So far, such a CRT as shown in FIG. 5(a) has been used, the surface of which has a single curvature. But an image displayed with the CRT has side-pin cushion distortion that is shown with solid line "a" in FIG. 6. An example of the correcting circuit which removes such a distortion is described in U.S. Pat. No. 3,906,305.
FIG. 5(b) shows a horizontal cross-sectional view of a non-spherical CRT, the screen of which has a plurality of curvatures, and they are smaller in the center, than with respect to both side portions of the screen. In other words, the radius of curvature for the central portion or area of the screen is larger than with respect to that of the side portions of the screen. The image that is displayed with such a CRT, has a side-pin cushion distortion as shown in "b" in FIG. 6. On the same non-spherical surface of such a CRT, a side line has a distortion as shown with "d" in FIG. 6. This type of side line distortion can be corrected with the field of a magnet which is arranged near the deflecting yoke. The magnetic field certainly corrects the side line distortion, but the side-pin cushion distortion has a form of "c" in FIG. 6. The side-pin cushion distortion shown with "b" and "c" in FIG. 6 cannot be corrected with the spool-type distortion correcting circuit of the above U.S. Patent, because they are different from the distortion of "a" in the same figure.